The yellow fever vaccine-17D (YFV-17D) is one of the most successful vaccines ever developed, yet we understand little of the mechanisms by which it induces protective immunity. Research during the previous cycle has focused on understanding the immunological mechanisms by which YFV-17D stimulates broad and long lasting T and B cell responses. The results of this work can be summed up thus: (i) YFV-17D stimulates multiple Toll-like receptors (TLRs) on dendritic cells (DCs), and this leads to robust CD8+ T cell responses and a mixed Th1/Th2 profile. Furthermore, YFV-17D induces the mammalian target of ramayicn (mTOR) in plasmacytoid DCs, which mediates type I IFN production, and enhances the CD8+ T cell response, (ii) A systems biological analysis of humans vaccinated with YFV-17D, demonstrated a robust type I IFN genomic signature in most vaccinees. Importantly, a distinct signature which included eukaryotic translation initiation factor 2 alpha kinase 4 [EIF2AK4, a stress response gene, was capable of prediciting with 90% accuracy the magnitide of the CD8+ T cell responses, (iii) Analysis of an individual who developed serious adverse events shortly after YFV-17D vaccination, revealed robust T and B cell responses, but polymorphisms in the CCR5 and RANTES genes, and greatly enhanced numbers of inflamatory monocytes. Taken together, these observations provide new insights into the mode of action of YFV-17D. The present proposal will follow up these observations mechanistically, and determine whether such responses are unique to YFV-17D, relative to the wild type strain Asibi, and the related flavivirus dengue. Aim 1: To determine whether stimulation of the integrated stress response by YFV-17D is necessary for the induction of antigen-specific CD8+ T cell responses. Aim 2: To evaluate the innate and adaptive immune responses stimulated by YFV-17D versus Asibi Aim 3: To determine the role(s) played by inflammatory monocytes and CCR5 in the pathogenesis of yellow fever and dengue. The successful completion of these aims will offer new insights into the mode of action of YFV-17D